1. Field of the Invention
The invention is generally directed to electron patterning, and more particularly electron patterning using scatter-nonscatter masking and even more particularly electron patterning in accordance with the SCALPEL process described in U.S. Pat. No. 5,079,112, issued Jan. 7, 1992. The present invention is also applicable to ion patterning.
2. Description of the Related Art
An exemplary conventional apparatus 40 depicted in FIG. 1 includes an electron or other energy source 41, condenser lens system 42, mask 43 including blocking regions 44 and transparent regions 45, objective lens 46, back focal plane filter 47 shown as provided with on-axis aperture 48, projector lens system 49, exposure medium 50, between lens 51 and stage 52, which together constitute registration and alignment system 53. The apparatus 40 is completed by vacuum chamber 54 and air lock 55, the latter providing the specimen exchange.
Another conventional apparatus shown in FIG. 2 includes a particle source 30, illustrated as an electron gun, delivering electron beam 31. Collimator lens 32 brings the initially diverging rays into parallel relationship at 33 as shown. Scan deflectors 34 and 35 are responsible for electronic scanning, e.g. with continuous x-direction scanning. The second deflector provides for y-direction movement, either continuous or as stepped intermediate x-direction scans. Mask 36, as depicted again in exemplary terms, is shown as segmented by struts 37. Upon passing through the mask, the now pattern-containing beam 38, comes under the influence of dynamic focus and stigmator deflectors 39 and 40. Deflectors 41 and 42 provide for a precision in placement of adjoining regions during x- and y-electronic scanning/stepping.
Projection lens 43 is provided with variable axis deflector 44. The multiple aperture filter 45 including apertures 46 produces a focused image on wafer 47 shown atop wafer stage 48. As discussed, for illustrative purposes, mask 36 is shown as constituted of pattern regions corresponding with strut-separated segments. Following modulation which imparts patterning information on the beam during passage through mask 36, the beam is converged, finally reaching a crossover (or image inversion) on or near the plane defined by aperture filter 46. The aperture filter 45 is included for electron imaging for blocking unwanted scattered radiation. It may serve, as well, to block other xe2x80x9cnoisexe2x80x9dxe2x80x94e.g. by blocking unwanted feature-edge scattered radiation. Aperture 46 may define the numerical aperture (or pupil) of the system.
The apparatus of FIG. 1 has separate condenser and projector lens systems. This may be preferred to facilitate focusing with minimum mechanical adjustment. There may be a further preference for multiple lenses in the projector system. For example, use of three lenses is useful to allow correction for image distortion and other aberrations, and to control image rotation as well.
The exemplary, conventional projector lenses 49, 43 may include other elements, e.g. may include a doublet of two optically equivalent lenses, in operation oppositely polarized to inherently cancel corresponding aberrations implicit to design or operation common to the two. Consistent with usual practice, the hardware responsible for generation of the functional shaped field is, itself, referred to as the xe2x80x9clensxe2x80x9d.
In FIG. 2, lenses 39 and 40 perform dynamic correction for aberration as well as for focusing, e.g. correcting for wafer height variation as well as field curvature. Assigning responsibility for dynamic adjustment to these lenses speeds the process by lessening inductive lag time. For example, dynamic aberration correction may entail additional deflectors compensating for errors resulting from equipment/process defects. Lenses 39 and 40 are illustrative and they may include additional elements.
FIGS. 3 and 4 illustrate systems with exemplary optics. FIGS. 3 and 4 discussed below illustrate schematics serving as a basis for discussing the fundamental principles involved in conventional electron patterning.
The conventional single lens system depicted in FIG. 3 makes use of beam electrons, or other delineating energy, identified as rays 1 incident on mask 2 which includes blocking regions 3 and transparent regions 4. Rays transmitted through transparent regions 4, are identified as rays 1a while those transmitted by blocking regions 3 are identified as rays 1b. Such rays are refracted by lens 5 with emerging rays made incident on back focal plane filter 6. As schematically depicted rays 1a pass through filter aperture 7 to result in image 9 including replicated illuminated regions 10 and unilluminated regions 11. Rays 1b scattered beyond a critical scattering angle, do not pass through aperture 7, but instead are absorbed or otherwise blocked by the non-apertured portion 8 of filter 6.
A conventional system in which scattered energy is selectively used to form the image is illustrated in FIG. 4. Here, scattered rays 1b pass through apertures 17 while transmitted rays 1a are now stopped by filter region 18. Image 19 a negative of image 9, results from selective illumination of regions 21. Regions 20 are unilluminated. In this arrangement, the back focal plane filter is absorbing although alternative designs may make use of forms of scattering such as Bragg scattering, etc.
Generally the projection system(s) illustrated in FIGS. 1-4 perform mask-to-wafer demagnification, on the order of 4:1-5:1. They are equally applicable to 1:1 and other magnification ratios.
Generally, systems illustrated in FIGS. 1-4 are arranged so that the resulting image on the wafer is focused as well as possible. As illustrated in a simplified system of FIG. 5, the goal is to focus the image 60 at the mask 62 on the wafer 64. Assuming the lens doublet 66, 68 as illustrated in FIG. 5 includes no aberrations, the best focused image 70 occurs at the Gaussian plane 72. However, due to several types of aberration, the most significant being chromatic and spherical aberrations, present in the lens doublet 66, 68, the best focused image plane 74 usually does not coincide with the Gaussian plane 72. Conventional techniques for determining the best focused image plane 74 have concentrated on separately analyzing the effect of chromatic and spherical aberrations on the location of the best focused image plane. These techniques are discussed below.
A conventional technique for estimating beam focus quality is a point spread function (PSF). Typically this includes launching up to 5000 trajectories"" from a single point at the center of the mask 62 plane and studying individual trajectories arrival points at the center of the wafer 64 plane.
Typical initial launch conditions included electrons"" angular distribution resulting in landing angles up to 12 mrad (up to 10 mrad, or up to 8 mrad), and electrons"" actual energy distribution. The latter have been obtained experimentally by analyzing electrons"" energy spread before and after passing through a mask 62 (silicon nitride SixNy, 0.1 xcexcm thick). An exemplary electron energy distribution is shown in FIG. 6.
As one can see from FIG. 6, electron passage through the mask 62 introduces an energy spread change: a substantial fraction (up to 20%) of electrons lose energy due to electron-plasmon inelastic collisions. Peak A in FIG. 6 describes electrons passed through the membrane without losing energy, and its width is defined by the beam source temperature. Peak B in FIG. 6 describes electrons passed through the membrane with substantial energy losses due to inelastic collisions with the plasmons. The energy distribution shown in FIG. 6 are characteristic of electron patterning masks, where membrane thickness is smaller than electron mean free path, such as SCALPEL(trademark) mask structures.
The data shown in FIG. 6 can be processed by simulation software developed to compute electron trajectories from the launch point through a projection column (in FIG. 5, the projection doublet 66, 68) down to the wafer 64 plane. Due to 3rd and 5th order geometrical aberrations of the lenses in FIG. 5, the image in the Gaussian plane 72 is no longer a point. Analysis has shown that chromatic and spherical aberrations are the most significant aberrations (axial objectxe2x80x94axial image). These aberrations create trajectories spread around a central point representing the Gaussian image, and PSF analysis can be used to estimate focus quality as xe2x80x9crise distancexe2x80x9d, or steepness, of the trajectories"" distribution about axis. It has been shown that PSF rise distance defines beam blur, or minimum resolvable feature size at the wafer 64.
It is well known in the art that magnetic and electrostatic lenses alike exert different focusing action on electrons having different energy. This effect is called chromatic aberration, and it means that electrons launched from the same axial point but having different energies will not be focused into a point in the image space, but instead cross the axis in different points, thus creating a circle of confusion at the ideal, or Gaussian image plane. In other words, chromatic aberration transforms a point object into a two-dimensional image. The same is true in reference to the electrons launched with different angles to the axis: magnetic and electrostatic lenses alike will focus larger angle electrons closer to the lens plane than small angle ones, thus creating circle of confusion at the ideal, or Gaussian image plane. In other words, spherical aberration also transforms a point object into a two-dimensional image.
The chromatic and spherical aberrations are described independently in terms of resulting circle of confusion or blur, a rough estimate of which can be obtained using a quadrature sum of individual circles of confusion diameters:
Dconfxcx9c(Dchr2+Dsh2)0.5
As stated above, PSF analysis computes actual electrons"" distributionxe2x80x94blurxe2x80x94at any given plane to estimate how close this distribution comes to the ideal, point-like Gaussian image.
PSF analysis may be used to find the plane in the image space where a circle of confusion reaches its minimum; the search criteria may be chosen as a radial distance between the boundaries drawn in such a manner that one of them represents 12% of total number of electrons launched, and another represents 88% of total number of electrons launched (12/88 criteria). The same procedure can be repeated for boundaries corresponding to 20% and 80% of total number of electrons launched (20/80 criteria). Conventional modeling results for 12/88, taking into account spherical aberration only, are shown in FIG. 7.
As one can see from FIG. 7, beam blur at the Gaussian plane (0.00 on the horizontal-axis) reaches 45 nm, while 4 microns in front of the Gaussian plane (xe2x88x924.00 on the horizontal-axis), the beam blur reaches its minimum at about 15 nm (14.528 nm). In other words, focus is best in the Z=xe2x88x924 micron plane.
The best focus plane location is also illustrated in FIG. 8, where a beam cross-section corresponding to FIG. 7 is shown. Again, focus is best in the Z=xe2x88x924 micron plane.
Likewise, when chromatic aberration only is taken into account, focus quality worsens due to a large fraction of slow electrons in the beam. This is illustrated in FIG. 9. Again, the best focus plane is located in 4 microns in front of the Gaussian plane. The corresponding beam cross-section profiles are shown in FIG. 10. As illustrated in FIG. 10, the minimum beam blur estimated as PSF rise distance is at about 60 nm (58.868 nm). The arrangements illustrated in FIGS. 7-10, which account for either spherical aberration only or chromatic aberration only do not accurately locate the best focus plane.
The present invention is directed to an apparatus for performing electron patterning and a method of focusing, wherein spherical aberrations are combined with chromatic aberrations, in order to lessen the negative effect of the chromatic aberrations. In one embodiment, the apparatus includes a source, a mask, a projection column, and a wafer on which the electron beam is to be focused, wherein the wafer is located in a plane where spherical aberrations of the projection column reduce negative defocusing effect caused by chromatic aberrations in the projection column. In another embodiment, the present invention is directed to a method of focussing where the spherical aberrations of the projection column reduce a negative defocusing effect caused by chromatic aberrations in the projection column.